1. Field of the Invention
The present invention is directed to improved light-sensitive positive photoresist compositions containing a quinonediazide polymer comprising the reaction product of a cresol-formaldehyde novolac resin and an o-quinonediazido compound (hereinafter referred to alternatively as "a modified novolac resin") and particularly to photoresist compositions containing a mixture of the quinonediazide polymer and a sulfonamide development enhancement agent.
2. Description of Related Art
Photoresist compositions are used in microlithography processes for making miniaturized electronic components such as in the fabrication of integrated circuits and printed wiring board circuitry. Generally, in these processes, a thin coating or film of a solution of the photoresist composition is first applied to a substrate material, such as silicon wafers used for making integrated circuits or aluminum or copper plates for printed wiring boards. The coated substrate then is baked to remove solvent and fix the coating onto the substrate.
The baked, coated surface of the substrate next is subjected to an image-wise exposure of radiation. This radiation exposure causes a chemical transformation in the exposed areas of the coated surface. Ultraviolet (UV) light and electron beam energy are radiation sources commonly used today in microlithographic processes. After this image-wise exposure, the coated substrate is treated with a developer solution to dissolve and remove either the radiation-exposed or the unexposed areas of the coated surface of the substrate.
There are two types of photoresist compositions: negative-working and positive-working. When negative-working photoresist compositions are exposed to radiation, the exposed areas of the resist composition become insoluble to a developer solution (e.g. a cross-linking reaction occurs), while the unexposed areas of the photoresist coating remain soluble. Thus, treatment of an exposed negative-working resist with developer removes non-exposed areas of the resist coating and creates a negative image in the photoresist coating. On the other hand, when positive-working photoresist compositions are exposed to radiation, those exposed areas of the resist composition become soluble to the developer solution (e.g. a decomposition reaction occurs), while those areas not exposed remain insoluble. Thus, treatment of an exposed positive-working resist with developer removes the exposed areas of the resist coating and creates a positive image in the photoresist coating.
After development, the partially unprotected substrate then is subjected to known subtractive or additive processes. For example, the substrate can be treated with a substrate-etchant solution, plasma gases or the like. Etchant solution or plasma gas etches that portion of the substrate where the photoresist coating was previously removed during development. Areas of the substrate where the photoresist coating still remains are protected during such subtractive, and also additive, processing. Thus, in the subtractive example identified above an etched pattern is created in the substrate material which corresponds to the exposure image of the actinic radiation. Later, remaining areas of the photoresist coating are removed during a stripping operation, leaving a clean etched substrate surface. This process, interlaced with other processing steps, then may be repeated numerous times to form the desired structure in the substrate.
As noted, the relief pattern of photoresist on a substrate produced by the method described above is useful for various applications including, for example, as an exposure mask or a pattern such as is employed in the manufacture of miniaturized integrated electronic components. The ability of a resist to reproduce very small dimensions, on the order of a micron or less, is extremely important in the production or large scale integrated circuits on silicon chips and similar components. Circuit density on such a chip can only be increased, assuming photolithographic techniques are utilized, by increasing the resolution capabilities of the resist.
Positive-working photoresist compositions recently have been favored over negative-working photoresists because the former generally have had better resolution and pattern transfer characteristics. Photoresist resolution is the lower limit of geometrically equal line and space patterns which the resist composition can transfer from a photomask to a substrate with a high degree of image edge acuity after exposure and development.
Particularly, with integrated circuits, miniaturization has proceeded with a rise in the integration level resulting in a demand for the formation of submicron patterns. At one time, a mask contact printing method had been used predominately for the formation of integrated circuits, but the resolution limit of this method was only about 2 .mu.m, and a reduction projection exposure method has since assumed a much larger use. This latter method involves reducing and projecting the pattern of a master mask (reticule) by passing light through a lens system. A submicron resolution can be obtained. This method has placed a much larger premium on photoresists which function at high resolution and over a relatively wide focus and exposure range. It also is highly desirable that the developed photoresist profile be nearly as vertical relative to the substrate surface as possible. Such demarcations between developed and undeveloped areas of the resist coating translate into more accurate pattern transfer of the mask image onto the substrate after etching. Improved processing latitude is particularly important where an intricate structure, formed by repeated exposures and etchings, is sought at the limits of miniaturization.
Several ingredients commonly make up positive photoresist compositions. A light stable, water-insoluble, alkali-soluble film-forming resin (or mixture of resins) is usually the major solid component. Phenol-formaldehyde novolacs, cresol-formaldehyde novolacs and poly(vinyl phenols) are well known examples of such resins. One or more light-sensitive compounds (also known as photoactive compounds or sensitizers) also are present in the photoresist composition. Naphthoquinonediazide compounds are examples of such sensitizers.
When a film is formed from the resin component, novolac resins are used most often, it is at least partially soluble in an aqueous alkaline developing solution. However, addition of the sensitizer inhibits dissolution of the film in the developing solution. When a substrate coated with a positive-working photoresist composition is subjected to an exposure of radiation, the sensitizer undergoes a radiation-induced chemical transformation in those exposed areas of the coating. This photochemical transformation eliminates the solubility-inhibiting property that the sensitizer had on the film-forming resin in alkaline developers. Accordingly, the radiation-exposed areas of the coating are now more soluble to aqueous alkaline developing solutions than the unexposed areas. The difference in solubility causes the exposed areas of the photoresist coating to dissolve when the coated substrate is immersed in the aqueous alkaline developing solution while the unexposed areas are largely unaffected, thus producing a positive relief pattern on the substrate.
It also is known to employ in positive-working resists, a light sensitive polymer containing recurring groups with pendant quinone diazide groups that are light sensitive. U.S. Pat. No. 3,046,120 discloses a polymer prepared by the condensation of o-benzoquinonediazido sulfonylchloride or o-naphthoquinonediazo sulfonylchloride with an alkali-soluble phenol-formaldehyde resin of the novolac type, or o- or m-cresol-formaldehyde resin. Another photosensitive polymer which has enjoyed wide use is the condensation product of a phenol-formaldehyde or cresol-formaldehyde resin and 1,2-diazo -5 sulfonate-naphthoquinone, capped with about 3-15 weight % of the naphthoquinonediazide. Unfortunately, when metal ion free developers are used to develop the exposed image, this photosensitive polymer tends to scum. Scumming involves a variety of observed phenomena that are characterized by an incomplete removal of the exposed resist film. Scumming is unacceptable in the fabrication of integrated circuits and printed wiring board circuitry. Such polymer compositions also have been used in resist compositions in combination with photoactive dissolution cohibitors (monomers) whose function is to thermally crosslink the polymer in non-exposed regions of the resist. Examples of such photoactive dissolution inhibitors are mentioned in U.S. Pat. No. 4,365,019 where a 1,3,5-trihydroxybenzene esterified with 1,2-naphthoquinonediazide sulfonic acid is disclosed and U.S. Pat. No. 4,684,597, where a 1,2,4-trihydroxybenzene esterified with a 1,2-naphthoquinonediazide sulfonic acid is disclosed.
U.S. Pat. No. 4,308,368 also describes a resist composition containing an adduct of a co-condensed novolac resin and a photosensitive o-quinonediazido compound. The novolac resin is prepared by reacting formaldehyde with phenol or cresol and a phenol substituted with an alkyl group having 3 to 12 carbon atoms or a phenyl group.
Positive-working photoresist compositions preferably contain other ingredients besides the film-forming resin and sensitizer. For example, one or more solvents may be added to facilitate application of the photoresist composition onto the substrate. Ethyl lactate, ethylene glycol methyl ether acetate, propylene glycol methyl ether acetate, N-methyl-2-pyrrolidone, and mixtures of cellosolve acetate, butyl acetate and xylenes are examples of commonly used solvents. The resin and sensitizer are dissolved in the solvent or solvents to facilitate their application (coating) to the substrate. Subsequent drying (baking) of the resist-coated substrate removes most of the solvent.
Other preferred photoresist additives include actinic and contrast dyes, anti-striation agents, plasticizers, speed enhancers and the like. Actinic dyes help provide increased resolution by inhibiting back scattering of light off the substrate. This back scattering causes the undesirable effect of optical notching, especially where the substrate surface is highly reflective or has topography. Contrast dyes enhance the visibility of developed images and may be used to facilitate pattern alignment during manufacturing. Anti-striation agents level out the photoresist coating or film to a uniform thickness. This is important to ensure uniform radiation exposure over the film surface. Plasticizers improve the coating and adhesion properties of the photoresist composition and better allow for the application of a thin coating or film of photoresist which is smooth and of uniform thickness onto the substrate. Speed enhancers generally tend to increase the solubility of the photoresist coating in both the exposed and unexposed areas. Thus, speed enhancers often are used in applications where speed of development is the overriding consideration even though some degree of contrast may be sacrificed, i.e. while the exposed areas of the photoresist coating will be dissolved more quickly by the developer, the speed enhancers will also cause a larger loss of photoresist coating from the unexposed areas.
The present invention overcomes many of the drawbacks of conventional photosensitive compositions of the type described in the prior art. The resist compositions of the present invention can be developed with metal ion free developers while avoiding scumming, exhibit excellent bleaching characteristics, and provide high contrast, high resolution and vertical sidewalls by retaining high resistance to aqueous alkali solutions over the unexposed area. The resist compositions simultaneously exhibit improved processing latitude, e.g. excellent focus and exposure latitude, with very small mask bias and excellent latent image stability. This reduces the effects on the resist composition of unavoidable process variations, resulting in more predictable critical dimensions and higher yields of high quality submicron devices. The resist compositions of the present invention also exhibit excellent plasma etch resistance.